Enhanced T_c and multiband superconductivity in the fully-gapped ReBe₂₂ superconductor

Abstract

Abstract In search of the origin of superconductivity (SC) in diluted rhenium superconductors and their significantly enhanced T c compared to pure Be (0.026 K), we investigated the intermetallic ReBe 22 compound, mostly by means of muon-spin rotation/relaxation ( μ SR). At a macroscopic level, its bulk SC (with T c = 9.4 K) was studied via electrical resistivity, magnetization, and heat-capacity measurements. The superfluid density, as determined from transverse-field μ SR and electronic specific-heat measurements, suggest that ReBe 22 is a fully-gapped superconductor with some multigap features. The larger gap value, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi mathvariant="normal">Δ</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> <mml:mrow> <mml:mi>l</mml:mi> </mml:mrow> </mml:msubsup> <mml:mo>=</mml:mo> <mml:mn>1.78</mml:mn> </mml:math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">k</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">B</mml:mi> </mml:mrow> </mml:msub> <mml:msub> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">c</mml:mi> </mml:mrow> </mml:msub> </mml:math> , with a weight of almost 90%, is slightly higher than that expected from the BCS theory in the weak-coupling case. The multigap feature, rather unusual for an almost elemental superconductor, is further supported by the field-dependent specific-heat coefficient, the temperature dependence of the upper critical field, as well as by electronic band-structure calculations. The absence of spontaneous magnetic fields below T c , as determined from zero-field μ SR measurements, indicates a preserved time-reversal symmetry in the superconducting state of ReBe 22 . In general, we find that a dramatic increase in the density of states at the Fermi level and an increase in the electron–phonon coupling strength, both contribute to the highly enhanced T c value of ReBe 22 .